Foam molding has a long history. A technique for obtaining a resin foamed product by injection molding is disclosed in, e.g., U.S. Pat. Nos. 3,268,639 and 3,384,691. In recent years, a foam molding method using a chemical or physical foaming agent can be learned from textbooks on synthetic resin molding.
Conventional foam molding has a very high foaming magnification, as is known in foamed styrol, and is accordingly excellent in weight reduction, but lacks mechanical strength. With a chemical foaming agent, a master batch scheme with which a thermally decomposable foaming material and a resin material are mixed immediately before molding is often employed. This leads to many problems such as harmfulness, mold corrosion, degradation in molding environment, and handling difficulty.
Regarding this, U.S. Pat. No. 3,796,779 proposes a foam which is obtained by blowing an inert gas such as carbon dioxide gas into a molten resin material directly so the gas is penetrated into the resin, and cooling the resultant resin. Since an inert gas is used as the foaming agent, harmfulness, mold corrosion, degradation in molding environment, handling difficulty, and the like are solved.
In U.S. Pat. No. 3,796,779 as well, since the gas is directly blown to the molten resin, the resin and the gas are not uniformly mixed, and an island structure with various cell shapes is formed. Then, the strength of the mixture becomes partially low. In this manner, it is very difficult to control the foaming state.
In order to solve these problems, a method of forming a very small foam called a microcell was found at Massachusetts Institute of Technology, USA early 1980. This method and an apparatus for it are disclosed in U.S. Pat. Nos. 4,473,665, 5,158,986, 5,160,674, 5,334,356, 5,571,848, and 5,866,053. According to the method and apparatus proposed by Massachusetts Institute of Technology, USA, a supercritical inert gas is blown to that portion of the plasticizer of an injection molder where a resin is to be fused. The sufficiently molten resin and the gas are mixed by a static mixer. The pressure and temperature are controlled. Consequently, a large number of small cells with a size of 25 μm or less are uniformly dispersed in a resultant foamed product. As the cell size is small, according to these references, a foamed product substantially free from strength degradation can be obtained.
To improve the quality of the outer appearance of the foamed product, the following method is known. That is, when a resin is to be injected into a mold, the mold is filled with a gas in advance so the interior of the mold is pressurized to be equal to or higher than the atmospheric pressure. The resin is injected to fill the mold completely. After that, the gas pressure applied to the interior of the mold is released, so the gas in the resin allows foaming upon pressure reduction.
According to the conventional method, the gas is directly blown into the molten resin material. When the gas is blown, that portion of the molten resin which comes into contact with the gas is quickly cooled. If the gas is blown continuously, a large part of the molten resin is cooled. As a result, the viscosity increases, and it takes time to restore a resin temperature and viscosity suitable for molding.
When the gas is heated to near the melt temperature of the resin in advance, as the temperature increases the volume of the gas increases. If the gas with the increased volume is blown into the resin, as the gas pressure in the resin is low, the foaming magnification after the resin is filled in the mold is very low.
In order to compensate for these defects, a method is available with which the temperature of the gas is increased and simultaneously the pressure is increased. While the gas concentration is maintained, the gas is blown into the molten resin. In this case, the pressure of the gas is very high, and the gas flows into the resin instantaneously when it is blown into the resin. Hence, it is difficult to control the gas blow amount. Since the gas is abruptly blown into the molten resin, the blown molten resin forms two separate layers of the gas and resin. To uniformly disperse the gas in the resin, kneading must be mechanically repeated by a static mixer or the like again. As a result, the apparatus becomes complicated, and the cycle is prolonged, so the productivity is impaired.
Originally, a plasticizer in an injection molder or extruder applies a certain degree of pressure to the molten resin in order to remove air in the material or during metering. If a gas is blown into the molten resin in the conventional manner and metering is performed, the blown gas may be undesirably discharged to the metering zone side of the plasticizer before it is dissolved in the resin completely.
According to the method described above, when the resin is to be injected into the mold, the mold is filled with the gas in advance so the interior of the mold is pressurized to be equal to or higher than the atmospheric pressure. After the resin is filled in the mold, the gas pressure is released. With this method, when the filling speed is high, the pressure of the gas filled in the mold cannot be controlled, and accordingly the filled gas serves as an obstacle to cause a short shot. When the filling speed of the resin is decreased, the gas that fills the mold can be controlled, so pressure control during and after resin filling can be performed. However, as the filling speed is low, that surface of the resin which is in contact with the mold is cooled by the mold and solidified to form a large skin layer. For this reason, in the foaming distribution of the foamed product, the foaming difference becomes very large between the surface and the center in the direction of thickness and between a portion in the vicinity of the gate and the finally filled portion.
According to the so-called master batch scheme, pellets containing a thermally decomposable chemical foaming material and pellets not containing a foaming material are mixed immediately before molding. With this scheme, it is very difficult to uniformly disperse the chemical foaming material during plasticization. Consequently, cells formed in the foamed product are dispersed nonuniformly, and a sufficient strength and precision cannot be obtained.